Electric heating devices



1967 R. D. BAXTER ETAL ELECTRIC HEATING DEVICES Filed June 30, 1964 United States Patent 3,296,496 ELECTRIC HEATING DEVIQES Robert D. Baxter and Thorhjorn Skogland, Sheihyville,

InrL, assignors to General Electric Company, a corporation of New York Filed June 30, 1964, Ser. No. 379,266 8 Claims. (Cl. 317-38) Our invention relates to electric heating devices and more particularly to sheathed electrical heating devices suitable for use as glow plugs in a diesel engine.

A commonly used sheathed electrical heating device employed as a glow plug consists of a helically wound resistance coil and a tubular metal sheath surrounding the coil. The resistance coil is embedded in a mass of compacted dielectric material, such as magnesium oxide, to electrically insulate the coil from the sheath and to provide for efiicient thermal conductivity between the heat generating portions of the coil and the sheath. Although such sheathed heating devices have been eminently successful as diesel engine glow plugs in applications where the range of voltage variations is not excessive, problems have been encountered in applications where the voltage applied across the glow plug undergoes a wide range of variation.

For example, in a diesel engine glow plug energized from a 12 volt battery source, the voltage available under sub-zero weather conditions is about volts during the heat-up period. After the starter switch is closed and the starting motor is energized from the battery source, the battery voltage falls off to about 7 volts. It is necessary that the glow plug supply suflicient heat at 7 volts to effect the ignition of the fuel and air mixture during the starting condition. When the engine fires and starts running, the starting motor is disconnected, and the generator or alternator now raises the voltage of the D.C. source to approximately 15 volts. For a short interval after the motor is started, the glow plug is energized until a smooth operation of the diesel engine is obtained. It will be understood that during this interval, the resistance coil of the glow plug must be capable of withstanding 15 volts.

Difficulties have been encountered in providing a sheathed heating device for use as a glow plug that is capable of supplying sufiicient heat at 7 volts during the cranking condition and that is also capable of withstanding without burning out the higher voltages applied to the glow plug after the engine starts running.

Accordingly, a general object of the present invention is to provide an improved heating device capable of being subjected to a wide range of operating voltages.

Another object of the present invention is to provide an improved heating device that is suitable for use as a diesel engine glow plug.

It is a more specific object of the invention to provide an improved diesel engine glow plug that is capable of generating sufficient heat to effect combustion of the fuel and air mixture under sub-zero weather conditions and that will not burn out when subjected to the generator or alternator voltage after the engine is started.

In accordance with one form of our invention we have provided an improved electric heating device having at least a first and a second resistance heating element surrounded by a first and a second metallic sheath. One end of the first and second resistance heating elements is joined in electrical circuit with one end of the first and 3,296,496 Patented Jan. 3, I967 second metallic sheaths, respectively. In order to provide a series connection between the resistance heating elements, a conductive member or bushing is provided for mounting the metallic sheaths and for joining the metallic sheaths in electrical circuit relation. A third terminal or electrode is attached to the conductive member to provide an electrical conection for selectively connecting only one of the resistance heating elements to the power source.

According to a more specific aspect of the invention, the improved heating device is adapted for use as a diesel engine glow plug. The bushing is provided with an externally threaded portion for engagement in a tap in the engine block or intake manifold. To electrically insulate the bushing from the engine block or manifold, the externally threaded portion is coatedwith an insulating material such as polytetrafiuorethylene resin. The third terminal is connected in circuit with a starter switch so that when the starter switch is actuated, only one of the resistance heating elements is connected across the D.C. source of the diesel engine electrical systems. With the improved glow plug arrangement, it is possible to connect two or more resistance heating elements across the D.C. source during the heat-up period, and during the cranking period to connect only one of the resistance heating elements in circuit with the D.C. source.

An advantage of the improved resistance heating element arrangement is that it is possible to employ resistance coils in applications such as diesel engine glow plugs where the device is subjected to a wide range of voltage variations.

The subject matter which we regard as our invention is set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may be better understood by referring to the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a partial sectional view of a sheathed heating device illustrating one embodiment of our invention wherein the heating device is utilized as a glow plug for a diesel engine;

FIGURE 2 is a fragmentary view of the threads of the bushing shown in FIGURE 1 enlarged to more clearly illustrate the resin insulation applied to the threads to insulate the bushing electrically from the engine block;

FIGURE 3 is a reduced side elevational view corresponding to the heating device shown in FIGURE 1 and embodying one form of the invention;

FIGURE 4 is a side elevational view of the electric heating device shown in FIGURE 3; and

FIGURE 5 is a simplified schematic circuit diagram of the. improved electric heating device and its external circuit connections in the electrical system of a diesel engine.

Referring now more particularly to FIGURES 1 through 4 of the drawing, there is shown an electric heating device in the form of a glow plug generally identified by the reference numeral 10. The glow plug 10 includes a pair of helically wound resistance coils 11,12, a pair of tubular metallic sheaths 13, 14 and a threaded bushing or conductive member 15 electrically joining the metallic sheaths 13, 1-4. Terminals 16, 17 and 18 are provided for connecting the resistance coils 11, 12 in circuit with a power source. At one end the resistance coils 11, 12 engage the ends of the metallic sheaths 13 and 14 respectively so that resistance coils 11, 12 are connected in electrical circuit with the metallic sheaths 13, 14. At the other ends the resistance coils 11, 12 are electrically connected to the terminals 16, 17 by means of the conductive rods 19 and 20 respectively. The resistance coils 11, 12 are embedded in a mass Olf compacted dielectric and thermally conductive material 22, 23 to insure good thermal conductivity between the coils 11, 12 and the metallic sheaths 13, 14.

To provide a suitable means for connecting the resistance coils 11 and 12 in an external circuit, the terminals 1-6, 17 are provided with pairs of nuts 24, 25 and 26, 27 and the washers 28, 29 respectively. As is seen in FIGURES 3 and 4, the third terminal 18 is electrically connected to the bushing 15 by brazing or other suitable means. This terminal also includes a means for making an electrical connection with an external lead and is comprised of a pair of nuts 30, 31 and a washer 32. As is shown in the fragmentary view of the portion of the bushing 15, the threads of the bushing 15 which engage an engine block or manifold (not shown), are electrically insulated from the bushing 15 by a suitable resin coating 33. In the illustrated embodiment of the invention the resin coating 33 was preferably a polytetrafluorethylene resin.

The open end of the sheath 19 is sealed by a compressed resilient washer 40 held in position by a split ring 41. A sealing disk 42 formed of suitable non-porous, electrically insulating and heat refractory material is attached to the end of the sheath 19. Similarly, sheath 20 is sealed by a compressed resilient washer 43 held in position by a split ring 44, and a sealing disk 45 is inserted in the open end of sheath 20 to close OK the end of sheath 20.

Turning now to the schematic circuit diagram shown in FIGURE 5, the glow plug 10 incorporating the improved heating-element arrangement is shown connected in circuit With a D.C. source 34, a starter switch 35 and a glow plug switch 36. The D.C. source 34 may be a battery supply, for example, having a nominal 12 or 24 volt rating. It will be seen that the resistance heating elements 11, 12 are connected at one end with the metallic sheaths 13 and 14 and at the other end with the terminals or electrodes 16, 17. In accordance with the improved heating element arrangement, the bushing 15, which serves to support the heating elements 11 and 12, also provides an electrical connection between the metallic sheaths 13, 14 so that when the glow plug switch 36 is closed, the two resistance heating elements 11, 12 are connected in series. When the starter switch 35 is depressed, only one resistance heating element 12 is connected in circuit across the D.C. source 34. When the starter switch 35 is in the position shown in FIGURE 5, and the glow plug switch 36 is closed, the path of current flow may be traced from the positive side of the D.C. source 34 through the upper contacts of the starter switch 35, lead 39, the terminal 16, resistance heating element 11, the metallic sheath 13, the bushing 15, the metallic sheath 14, the resistance heating element 12, and the glow plug switch 36 to the negative side of the D.C. source 34.

Having more specific reference to FIGURE 5, we will now more fully describe the operation circuit shown therein as adapted for use in a diesel engine electrical system having a battery with a 12 volt nominal rating. Let us assume that the engine is being operated under sub-zero weather conditions and that the voltage available for the glow plug 10 during the heat-up period of the engine is approximately 10 volts. When the glow plug switch 36 is initially closed during the initial heat-up period, which may range from 20 to seconds, the resistance heating elements 1 1 and 12 are connected in series across 10 volts. During the heat-up period, the voltage across the D.C. source drops to 7 volts. 5 volts. At the end of the heat-up period, the starter switch is depressed to energize the starting motor (not shown). When the starter motor is energized, the voltage across the D.C. source drops to 7 volts.

In accordance with the improved arrangement, when the starter switch 35 is depressed, resistance heating element 11 is cut out 0f the circuit and only one heating element 12 is connected across the D.C. source. The 7 volts available during the cranking period is applied only across one heating element 11. The single resistance element 11 supplies sufficient heat energy to effect the ignition of the fuel mixture. When the engine fires and starts running, the starting motor is disconnected from the engine and the D.C. source. The generator now raises the D.C. source voltage 15 volts. With the starter switch 35 returned to its initial position, it will be seen in FIGURE 5 that both of the resistance elements 11 and 12 are again connected in series so that approximately 7.5 volts is applied across each element. Thus, in the exemplification of the invention, the voltage range on the resistance heating elements 11 and 12 was between 5 and 7.5 volts. It will be appreciated that a conventional resistance heating element in the above-described application would be subjected to a voltage range between 7 and 15 volts.

Although the illustrated embodiment of the invention has been described in connection with a specific application to a diesel engine electrical system, it will be appreciated that the advantages of the present invention can be realized in other heating device applications. An important advantage of the improved resistance heating element arrangement is that the heating device can be effectively utilized in applications where an appreciable variation occurs in the output voltage from which the electrical device is energized.

It would be apparent from the foregoing description that the improved heating devices can be utilized in applications where the voltage supply variations are considered excessive and might impair the service life of conventional resistance heating devices because of the inherent voltage limitations of the heating coils. While we have shown and described what at present is considered to be the preferred embodiment of our invention in accordance with the patent statutes, it will be obvious to those skilled in the art that many changes and modifications may be made in the disclosed structure without departing from the invention. For example, although only two heating elements are used in the illustrative embodiment of the invention, it will be apparent that the advantages of the invention may be achieved in a heating device employing more than two resistance elements. It is, therefore, intended in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electric heating device comprising a first and a second helical resistance heating element, a first and a second metallic sheath each having a closed end and surrounding said first and second helical resistance heating elements respectively, said first and second helical resistance elements joined electrically in circuit at one end respectively generally adjacent the closed ends of said first and second metallic sheaths, a first and second terminal connected electrically in circuit with the other ends of said first and second helical resistance elements, powdered heat refractory material disposed between said helical resistance heating elements and said metallic sheaths to provide a heat conducting path from said helical resistance elements to said metallic sheaths, a conductive member engaging said first and second metallic sheaths and thereby to provide a series electrical connection between said resistance heating elements, and a third terminal joined with said conductive member for providing an electrical connection for selectively connecting only one of said resistance heating elements to the power source.

2. An electric heating device comprising a first and a second electric resistance heating element, a first and second hollow metallic sheath each having a closed end and surrounding said first and'second electric resistance heating elements respectively, one end of said first and second electric resistance heating elements connected electrically with said closed end of said first and second hollow metallic sheaths, a first and a second electrical terminal for connection with a power source, means connecting the other ends of said first and second electric resistance heating elements respectively in circuit with said first and second terminals, heat transmitting means disposed between each of said electric resistance heating elements and the metallic sheaths for conducting heat from the resistance heating elements to the sheaths, a conductive member joining said sheaths in electrical circuit to provide a series connection between said first and second resistance heating elements, a third terminal joined in electrical circuit with said conductive member for connection with a switching means to permit only one of said electric resistance heating elements to be selectively connected across the power source when one of said first and second terminals and the third terminal are energized from the power source.

3. An electric heating device comprising a first and second elongated electric resistance heating element, a first and second hollow metallic sheath respectively surrounding said first and second electric resistance heating elements and joined respectively with one end of the first and second electric resistance heating elements, heat transmitting means disposed between each electric resistance heating elements andits surrounding sheath, each said sheath including closure means for retaining said heat transmitting means in place, a first and a second terminal for connection with a power source and joined respectively in electrical circuit with the other ends of said first and second electric resistance heating elements, a conductive member engaging said metallic sheaths to provide a series electrical connection therebetween and thereby place the resistance heating elements in series across said first and second terminals, and a third terminal joined with said conductive member for providing an electrical connection in an external circuit for selectively causing only one of said resistance heating elements to be connected to the power source.

4. An electric glow plug comprising a first and a second resistance heating element, a first metallic sheath having at least one closed end and disposed around said first resistance heating element and in electrical engagement with one end of said first resistance heating element adjacent the closed end thereof, a second metallic sheath having at least one closed end and disposed around said second resistance heating element and in electrical engagement with one end of said second resistance heating element adjacent the closed end thereof, a first and a second terminal for connection in circuit with a power source, said first terminal joined in electrical circuit with the other end of said first resistance heating element, said second terminal joined in electrical circuit with the other end of said second resistance heating element, a conductive bushing formed with a pair of apertures, said metallic sheaths being disposed in said apertures and in electrical engagement with said conductive bushing, said conductive bushing providing a series electrical connection between said metallic sheaths to thereby join said first and second resistance heating elements in series circuit relation across the first and second terminals, and a third terminal joined in electrical engagement with said conductive bushing to provide a means for connecting said series electrical connection in an external circuit for selectively disconnecting one of said resistance heating elements from the power source.

5. An electric glow plug comprising at least a first and a second electric resistance heating coil, a first hollow metallic sheath having a closed end and surrounding said first resistance heating coil and joined electrically adjacent said closed end with one end of said first resistance heating coil, a second hollow metallic sheath having a closed end and surrounding said second resistance heating coil and joined electrically adjacent said closed end with one end of said second resistance heating coil, heat transmitting means disposed between each of said resistance heating coils and the surrounding sheath, a first terminal for connection in circuit with a power source and joined with the other end of said first resistance heating coil, a second terminal for connection in circuit with the power source and joined with the other end or said second resistance heating coil, an electrically conductive member engaged with said first and second metallic sheaths for providing a series electrical connection between the first and second metallic sheaths thereby to place the resistance heating coils in series circuit relation across the first and second terminals, and a third terminal joined with said conductive member for providing an electrical connection in an external circuit for selectively causing at least one of the resistance heating coils to be disconnected from the power source.

6. An electric glow plug for use in a diesel engine, said electric glow plug comprising; a first and a second resistance heating element, a first hollow metallic sheath surrounding said first resistance heating element and electrically engaging one end of said first resistance heating element, a second hollow metallic sheath surrounding said second resistance heating element and electrically engaging one end of said second resistance heating element, a first and a second terminal for connection in circuit with a power source, means connecting the other end of said first electric resistance heating element in electrical circuit with said first terminal, means also connecting the other end of said second resistance heating element in electrical circuit with said second terminal, heat transmitting means disposed between each of said resistance heating elements and said hollow metallic sheaths for conducting heat from the resistance heating elements to the metallic sheaths, a conductive bushing joining said first and second metallic sheaths in electrical circuit to provide a series connection between said first and second resistance heating elements, means for insulating said conductor bushing from the diesel engine, and a third terminal joined in electrical circuit with said conductive bushing for connection in an external circuit to permit only one of the electric resistance heating elements to be connected across the power source during the cranking condition of the diesel engine.

7. The electric glow plug set forth in claim 6 wherein said conductive bushing is formed with an externally threaded portion and said insulating means is comprised of a coating of polytetrafluorethylene resin.

8. An electric circuit comprising: a DC. source; a glow plug including a first and a second resistance heating element, a first metallic sheath having a closed end, said first metallic sheath surrounding said first resistance heat ing element and electrically engaging one end of said first resistance heating element adjacent said closed end, a second hollow metallic sheath having a closed end, said second metallic sheath surrounding said second resistance heating element and electrically engaging one end of said second resistance heating element adjacent said closed end, a first terminal connected in circuit with the other end of said first electric resistance heating element and a second terminal connected in circuit with the other end of said second electrical resistance heating element, an electrically conducting bushing joining said first and second metallic sheaths in electrical circuit to provide a series connection between said first and second resistance heating elements, and a third terminal joined in electrical circuit with said conductive bushing; circuit means including a first switch for connecting said first terminal to one side of said D.C. source, said first switch when opened disconnecting said glow plug from the power source; and circuit means connecting said second and third terminals in circuit with the other side of said D.C.

7 8 source and including a second switch, said second switch 2,575,113 .11/1951 Lennox 31783 being operable between a first position and a second 2 34 920 5 1959 Moule et 1 123 osition, said switch when in said first osition connectin Said second terminal in circuit with th: other side of m 2933896 4/1960 Fen-1e 60 39'82 power source and when actuated to said second position 5 FOREIGN PATENTS causing said third terminal to be connected in circuit with said other side of the power source and causing said 839,580 5/ 1952 Germany. second terminal to be disconnected thereby deenergizing 575,569 4/1958 Italy one of the resistance heating elements. I

1O RICHARD M. WOOD, Primary Examiner. References Clted by the Exammer V. Y. MAYEWSKY, Assistant Examiner.

UNITED STATES PATENTS 1,704,934 3/1929 Briggs.

2,130,365 9/1938 Paulson 338-24O X 15 

1. AN ELECTRIC HEATING DEVICE COMPRISING A FIRST AND A SECOND HELICAL RESISTANCE HEATING ELEMENT, A FIRST AND A SECOND METALLIC SHEATH EACH HAVING A CLOSED END AND SURROUNDING SAID FIRST AND SECOND HELICAL RESISTANCE HEATING ELEMENTS RESPECTIVELY, SAID FIRST AND SECOND HELICAL RESISTANCE ELEMENTS JOINED ELECTRICALLY IN CIRCUIT AT ONE END RESPECTIVELY GENERALLY ADJACENT THE CLOSED ENDS OF SAID FIRST AND SECOND METALLIC SHEATHS, A FIRST AND SECOND TERMINAL CONNECTED ELECTRICALLY IN CIRCUIT WITH THE OTHER ENDS OF SAID FIRST AND SECOND HELICAL RESISTANCE ELEMENTS, POWDERED HEAT REFRACTORY MATERIAL DISPOSED BETWEEN SAID HELICAL RESISTANCE HEATING ELEMENTS AND SAID METALLIC SHEATHS TO PROVIDE A HEAT CONDUCTING PATH FROM SAID HELICAL RESISTANCE ELEMENTS TO SAID METALLIC SHEATHS, A CONDUCTIVE MEMBER ENGAGING SAID FIRST AND SECOND METALLIC SHEATHS AND THEREBY TO PROVIDE A SERIES ELECTRICAL CONNECTION BETWEEN SAID RESISTANCE HEATING ELEMENTS, AND A THIRD TERMINAL JOINED WITH SAID CONDUCTIVE MEMBER FOR PROVIDING AN ELECTRICAL CONNECTION FOR SELECTIVELY CONNECTING ONLY ONE OF SAID RESISTANCE HEATING ELEMENTS TO THE POWER SOURCE. 